The recycling of copper from copper scrap has become more and more important due to the increased value of copper metal. A considerable quantity of copper scrap is available from the electrical cable industry. Typically, such scrap often contains lead, tin and antimony as major impurities in the scrap due to the various solder materials used in the fabrication of the cable. It is often extremely important to remove these impurities from the copper scrap before the copper can be used again.
Generally, the efficient removal of lead from the copper is accomplished in a separate step from the removal of impurities such as the tin and antimony. Typically, the tin and antimony content in copper should be reduced to below 50 ppm and 5 ppm respectively, and preferably to below 10 ppm or 3 ppm, respectively. One method used in the industry to remove tin from scrap copper is to charge a furnace which contains copper having a large tin contamination, for example, 0.3% tin or more, melting the copper to be refined and blowing oxygen into the copper to an oxygen percentage of about 1% and then treating the molten copper with silica sand. After this treatment, the copper bath is poled to remove practically all of the oxygen. The melt is then treated with a few tenths of a percent of soda ash. The bath is then cleaned, reoxidized to about 1% oxygen and again treated repeatably with silica sand until the tin content is reduced to the required limits. Alternatively, one can make additions of sodium carbonate and sodium nitrate to the bath under high oxygen conditions. This purification technique generally takes approximately two days and results in a copper loss to the slag of between 5 and 10 percent. In a paper reported in the Journal of the Institute of Metals, 86, 59-64, (1957-58) J. E. Stolarczyk et al. reported several fluxes useful for the removal of tin from copper. These fluxes included 50/50 mixtures of Cu.sub.2 O and Na.sub.2 Co.sub.3, Fe.sub.2 O.sub.3 and SiO.sub.2 a 30/70 mixture of CaO and Na.sub.2 Co.sub.3, a 30/35/35 weight percent mixture of CaO, FeO and SiO.sub.2 and a 60/20/20 weight percent mixture of Cu.sub.2 O, CaO and Al.sub.2 O.sub.3. In U.S. Pat. No. 3,561,952 there is taught the use of alkali oxide-silicate slags as well as alkali oxide phosphates and/or borates for the removal of lead and tin from copper scrap. In U.S. Pat. No. 3,262,773 a dry refining process for the removal of arsenic, antimony, tin and other acidic oxide forming elements from molten copper is presented by combining the acid oxides of such elements with basic materials such as alkaline earth oxides, in particular calcium fluoride and calcium oxide in the slag. This patent suggests the use of slags having a final composition containing from 35 to 45 weight percent lime and wherein the slag is subjected to oxidizing treatment before the addition thereto of calcium oxide or calcium oxide precursers. The initial slag content in accordance with the teachings of this patent comprise 4 to 12 weight percent calcium oxide based upon the weight of the copper to be refined together with fluorspar (calcium fluoride) so as to form a slag phase having an excess of calcium oxide, oxidizing the copper to at least 1% oxygen content and contacting the slag phase with the molten crude copper so as to remove the impurities from the copper.
While the above methods have been reported and are available for the removal of these impurities from molten copper scrap, further improvements are desirable.